1. Field of the Invention
This invention relates to an improved electrical cable and process for making the subject cable having a low dielectric constant, and in particular, a flexible shielded ribbon cable having multiple conductors with improved transmission line characteristics, improved crush resistance and good mechanical characteristics for mass termination.
2. Description of Prior Art
There already exist in the marketplace multiconductor flexible, mass terminable cables having transmission line characteristics such as controlled impedance, crosstalk, propagation delay, etc. It is well known that by lowering the effective dielectric constant of the cable by including air in the dielectric, the signal speed can be increased.
Providing porosity in a dielectric suitable for cables is known. Foam type insulations are known from U.S. Pat. No. 3,529,340, where the foam coated conductors were placed in a sheath which is shrunk onto the foam covered conductors. Another patent is U.S. Pat. No. 4,680,423, disclosing a foam insulation said to contain a large percentage of air trapped within the material. In this patent, foam covered conductors are embedded within an insulating material which completely surrounds the foam insulation. The insulating material is used to hold the conductors in a parallel configuration and provides strength to the cable when subjected to compression.
Another patent describing a foamed insulative material for conductors includes U.S. Pat. No. 5,110,998, issued May 5, 1992 describing an ultramicrocellular foamed polymer structure formed from suitable polymers including the class of synthetic, crystalline and crystallizable, organic polymers, e.g. polyhydrocarbons such as linear polyethylene, polypropylene, stereo-regular polypropylenes or polystyrene, polyethers such as polyvinylidene fluoride, polyamides both aliphatic and aromatic, and the list goes on, but concludes the polymers should have a softening point of at least about 40.degree. C. The high degree of orientation of the closed polyhedral cells, of this foamed material, contributes to the strength of the structures. The foamed structure is described for use as an insulative material for individual conductors smaller than 1.27 mm and annular insulation thickness less than 0.51 mm. The insulative material is flash spun over a moving wire in air at ambient temperature and pressure or by an extrusion spinning method. The crush resistance of the material is described in column 3 lines 64 to column 4 line 9. The recovery rate is not considered sufficient to provide good electrical properties to signal wire and the material is not suitable for making ribbon cable.
Further, W. L. Gore & Associates, Inc. sells cable made with "Gortex".TM." dielectric films, a porous polytetrafluoroethylene (PTFE). Polytetrafluoroethylene is not a conventional thermoplastic and is not easily processed and is costly. Various patents have been assigned to W. L. Gore & Associates, Inc. of Newark, Del. including U.S. Pat. Nos. 3,953,566 and 4,187,390 relating to the process for making a porous polytetrafluoroethylene polymer; 4,443,657 which relates to the manufacture of a ribbon cable using two layers of polytetrafluoroethylene (PTFE) as insulation, and 4,866,212 relating a coaxial electric cable formed of an expanded polytetrafluoroethylene.
U.S. Pat. No. 4,475,006 describes a shielded ribbon cable comprising a plurality of conductors encased in a low-loss plastic or elastomer insulation such as polyethylene, polypropylene, polyurethane, tetrafluoroethylene polymer, fluorinated ethylene propylene and EPDM rubber, and a shield wrapped around the cable and adhered to the insulation. The shield material preferably had a maximum resistivity (minimum conductivity) of 3.5 milliohms per square and examples of the shielding material included a copper foil, an aluminum foil/polyester laminate or an expanded copper foil mesh. The shield was cigarette wrapped about the insulation with the shield bonded to the insulation to provide an effective uniform transverse and longitudinal dielectric constant.
Another patent which teaches the construction of a shielded ribbon cable is U.S. Pat. No. 4,533,784 which describes an electrical shield having a continuous metallic foil having a plurality of transverse folds to provide a shielded cable with greater flexibility and less subject to cracking. This patent discloses one type of shielding material usable for the cable of the present invention.
High speed cables of the prior art generally utilize expanded PTFE dielectrics such as those sold by W. L. Gore & Associates, Inc. or foamed perfluoro polymers. Such cable structures have reduced crush resistance as compared to solid dielectrics. This reduced crush resistance results in reduced transmission line characteristics as a result of damage caused by normal routing or handling of cables made from these conventional dielectrics.
Because of the very high processing temperatures, cables made in ribbon format with polytetrafluoroethylene generally have silver plated or nickel plated conductors to avoid the oxidation of the conductors during processing. Use of either causes significant cost increase. In addition, if nickel is used, difficulty in soldering to the conductors is encountered.
It should be noted that lamination and fusion of thermoplastic insulations to make ribbon cables has been taught in the prior art such as U.S. Pat. No. 3,523,844 assigned to David J. Crimmins, et. al. and U.S. Pat. No. 2,952,728 assigned to Kyohei Yokose, et. al. The Crimmins patent teaches lamination of solid dielectrics around variably spaced wires. This method will not work with air filled dielectrics without collapsing the air filled structure. Similarly, The Yokose patent teaches lamination of solid dielectrics around conductors. However, the tool or roller design employed will cause excessive melting and destruction of the fibril structure of the material in the present invention. Both of the methods employed in the prior art would not work with the materials presented herein. The process and materials of the present invention teach lamination without significant destruction or collapse of the air filled structure adjacent the conductors.
U.S. Pat. No. 4,443,657, assigned to W. L. Gore & Associates, Inc., demonstrates a means of bonding sheets of PTFE using a sintering process. The softness of the unsintered core dielectric forces the inventor to place a solid layer of insulation over the top of the unsintered core resulting in significant reductions in electrical performance of the finished cable due to the solid dielectric.
The crush resistance of dielectrics which contain large percentages of air voids has long been a problem in the use of high speed dielectrics. In U.S. Pat. No. 4,730,088 assigned to Junkosha Co., LTD., Japan, expanded polytetrafluoroethylene (PTFE) was reinforced by use of a laser beam or a hot metal rod. The piercing of the soft insulation by the beam or rod caused a unique phenonema to occur to the porous PTFE called sintering. In this case, the sintering causes the soft dielectric to form a solid skin of PTFE on the inside wall of the created hole. Since sintered PTFE has many times the structural strength of the unsintered porous dielectric, the cylinders so created act like beams to resist crushing forces. An alternate method disclosed, used heated rolls to put grooves in the surface of the insulation. Both methods sole purpose is to increase the crush resistance of the insulation. Both disclosed solutions suffer from the creation of discontinuities in the dielectric which add to signal speed variation as the electrical fields encounter these discontinuities.
The product disclosed in the present application also has improved crush resistance over unsintered expanded polytetrafluoroethylene without the time consuming and expensive process of forming sintered cylinders or grooves in the dielectric. This product, in addition to having the improved electrical properties at substantially reduced cost and with improved crush resistance, does not have the dielectric discontinuities associated with the formation of sintered shapes as with prior art. The process used to form this product also can be accomplished at substantially reduced temperatures permitting conductors to be used with or without plating which provides additional cost reduction. The unique crush resistant properties of the subject product result since the polymers employed to make the insulation do not have the uncharacteristic changes called sintering but rather have the improved properties immediately upon cooling thus eliminating costly and time consuming sintering processes.
The prior art demonstrates that many attempts have been made to provide electrical cables with lower dielectric constant and/or fixed shield-wire spacing to improve electrical characteristics. The prior art cables sacrifice durability and crush resistance to achieve lower dielectric constant and faster propagation velocities. This is in part due to the necessity to employ polymer structures which are inherently soft or weak in their structural integrity. Examples being the foamed materials and porous polytetrafluoroethylene polymer.
The present invention provides an improved cable construction which can have lower dielectric constants and higher propagation velocities and maintain the same uniformity along the cable, even though it is flexed since the dielectric is more crush resistant and the shield is maintained in spaced position at the areas where the cable is flexed. In addition the processing of the product is done at lower temperature permitting the use of conductors with or without plating.